arXiv subjects

G. Jackson

Publications and source records attributed to G. Jackson.

15 recordsLinked to original sources

arXiv2026

Collisional energy loss distribution of a fast parton in a hot or dense QCD medium

We compute the probability distribution for collisional energy loss of an ultrarelativistic parton crossing a quark-gluon plasma. This collisional quenching weight has not been determined previously, unlike the average collisional loss per unit distance, although it should be a more accurate quantity to use in jet-quenching phenomenology. The quenching weight is obtained from a well-known kinetic equation which resums an arbitrary number of elastic scatterings of the energetic parton with the medium, providing a complete description of the stochastic energy exchange, including the possibility of energy gain from thermal fluctuations. The formulation also naturally extends the standard treatment of collisional energy loss to finite path lengths, which could be relevant not only for heavy-ion collisions, but also for light-ion, and possibly proton-nucleus and proton-proton collisions. We predict the quenching weight in a setup where individual elastic scatterings are described using the hard thermal loop approximation for soft exchanges, with a smooth matching to the hard domain.

hep-ph↗

arXiv2025

Fast and Flexible Neutrino Decoupling Part I: The Standard Model

Cosmological determinations of the number of relativistic neutrino species, $N^{ }_{\rm eff}$, are becoming increasingly accurate, and further improvements are expected both from CMB and BBN data. Given this context, we update the evaluation of $N^{ }_{\rm eff}$ and the current entropy density via the momentum-averaged approach. This allows for a numerically fast description of neutrino decoupling, easily portable to an array of new physics scenarios. We revisit all aspects of this approach, including collision terms with full electron mass dependence, finite temperature QED corrections to the equation of state, neutrino oscillations, and the modelling of neutrino ensembles with effective chemical potentials. For integrated observables, our results differ by less than $0.04\%$ from the solution of the momentum-dependent evolution equation. We outline how to extend the approach to BSM settings, and will highlight its power in Part II. To facilitate the practical implementation, we release a Mathematica and Python code within nudec_BSM_v2, easily linkable to BBN codes.

hep-ph↗

arXiv2024

$\nu\bar\nu$ production, annihilation, and scattering at MeV temperatures and NLO accuracy

Interaction rates of neutrinos and antineutrinos within a QED plasma determine the dynamics of their decoupling in the early universe. We show how to define the relevant double-differential production, annihilation, and scattering rates at NLO. Integrating over these rates with specific weights, other quantities from the literature can be obtained, such as energy transfer rates, or a neutrino interaction rate. In the limit of massless electrons, we show that NLO corrections to the energy transfer rates are as small as those that enter the previously determined neutrino interaction rate, and only have a small influence on the neutrino decoupling parameter, $N_{\rm eff}\,$. For comparison, the influence of a finite electron mass is quantified at LO. Finally we provide a tabulation and fast interpolation routine for all double-differential rates, in order to allow for their use in non-approximate kinetic equations, which may further reduce the systematic uncertainties of the Standard Model prediction for $N_{\rm eff}\,$.

hep-ph↗

arXiv2023

QED corrections to the thermal neutrino interaction rate

Motivated by precision computations of neutrino decoupling at MeV temperatures, we show how QED corrections to the thermal neutrino interaction rate can be related to the electron-positron spectral function as well as an effective $\bar{\nu}\nu\gamma$ vertex. The spectral function is needed both in a timelike and in a spacelike domain, and for both of its physical polarization states (transverse and longitudinal with respect to spatial momentum). Incorporating an NLO evaluation of this spectral function, an estimate of the $\bar{\nu}\nu\gamma$ vertex, and HTL resummation of scatterings mediated by soft Bose-enhanced $t$-channel photons, we compute the interaction rate as a function of the neutrino momentum and flavour. Effects on the $ -(0...2)\%$ level are found, noticeably smaller than a previous estimate of a related quantity.

hep-ph↗

arXiv2021

Efficient numerical integration of thermal interaction rates

In many problems in particle cosmology, interaction rates are dominated by ${2}\leftrightarrow{2}$ scatterings, or get a substantial contribution from them, given that ${1}\leftrightarrow{2}$ and ${1}\leftrightarrow{3}$ reactions are phase-space suppressed. We describe an algorithm to represent, regularize, and evaluate a class of thermal ${2}\leftrightarrow{2}$ and ${1}\leftrightarrow{3}$ interaction rates for general momenta, masses, chemical potentials, and helicity projections. A key ingredient is an automated inclusion of virtual corrections to ${1}\leftrightarrow{2}$ scatterings, which eliminate logarithmic and double-logarithmic IR divergences from the real ${2}\leftrightarrow{2}$ and ${1}\leftrightarrow{3}$ processes. We also review thermal and chemical potential induced contributions that require resummation if plasma particles are ultrarelativistic.

hep-ph↗

arXiv2020

Gravitational wave background from Standard Model physics: Complete leading order

We compute the production rate of the energy density carried by gravitational waves emitted by a Standard Model plasma in thermal equilibrium, consistently to leading order in coupling constants for momenta $k\sim \pi T$. Summing up the contributions from the full history of the universe, the highest temperature of the radiation epoch can be constrained by the so-called $N_{\rm eff}$ parameter. The current theoretical uncertainty $\Delta N_{\rm eff} \le 10^{-3}$ corresponds to $T_{\rm max} \le 2\times 10^{17}$ GeV. In the course of the computation, we show how a subpart of the production rate can be determined with the help of standard packages, even if subsequently an IR subtraction and thermal resummation need to be implemented.

hep-ph↗

arXiv2019

A thermal neutrino interaction rate at NLO

The interaction rate of an ultrarelativistic active neutrino at a temperature below the electroweak crossover plays a role in leptogenesis scenarios based on oscillations between active neutrinos and GeV-scale sterile neutrinos. By making use of a Euclideanization property of a thermal light-cone correlator, we determine the $O(g)$ correction to such an interaction rate in the high-temperature limit $\pi T \gg m_W$, finding a $\sim 15 ... 40\%$ reduction. For a benchmark point, this NLO correction decreases the lepton asymmetries produced by $\sim 1\%$.

hep-ph↗

arXiv2019

Testing thermal photon and dilepton rates

We confront the thermal NLO vector spectral function (both the transverse and longitudinal channel with respect to spatial momentum, both above and below the light cone) with continuum-extrapolated lattice data (both quenched and with $N_{\rm f} = 2$, at $T \sim 1.2 T_{\rm c}$). The perturbative side incorporates new results, whose main features are summarized. The resolution of the lattice data is good enough to constrain the scale choice of $\alpha_{\rm s}$ on the perturbative side. The comparison supports the previous indication that the true spectral function falls below the resummed NLO one in a substantial frequency domain. Our results may help to scrutinize direct spectral reconstruction attempts from lattice QCD.

hep-ph↗

arXiv2018

Hydrodynamic fluctuations from a weakly coupled scalar field

Studies of non-equilibrium dynamics of cosmological phase transitions may involve a scalar field interacting weakly with the energy-momentum tensor of a thermal plasma. At late times, when the scalar field is approaching equilibrium, it experiences both damping and thermal fluctuations. We show that thermal fluctuations induce a shear viscosity and a gravitational wave production rate, and propose that including this tunable contribution may help in calibrating the measurement of the gravitational wave production rate in hydrodynamic simulations. Furthermore it may enrich their physical scope, permitting in particular for a study of the instability of growing bubbles.

hep-ph↗

arXiv2016

Tracing the pressure of the gluon plasma

Being interested in how a strongly coupled system approaches asymptotic freedom, we re-examine existing precision lattice QCD results for thermodynamic properties of the gluon plasma in a large temperature range. We discuss and thoroughly test the applicability of perturbative results, on which grounds we then infer that the pressure and other bulk properties approach the free limit somewhat slower than previously thought. We also revise the value of the first non-perturbative coefficient in the weak-coupling expansion.

hep-ph↗

arXiv2012

Evidence for the role of fluctuations in the thermodynamics of nanoscale drops and the implications in computations of the surface tension

Test area deformations are used to analyse vapour-liquid interfaces of Lennard-Jones particles by molecular dynamics simulation. For planar vapour-liquid interfaces the change in free energy is captured by the average of the corresponding change in energy, the leading-order contribution. This is consistent with the commonly used mechanical (pressure tensor) route for the surface tension. By contrast for liquid drops one finds a large second-order contribution associated with fluctuations in energy. Both the first- and second-order terms make comparable contributions, invalidating the mechanical relation for the surface tension of small drops. The latter is seen to increase above the planar value for drop radii of ~8 particle diameters, followed by an apparent weak maximum and slow decay to the planar limit, consistent with a small negative Tolman length.

cond-mat.soft↗

arXiv2010

Cholesteric order in systems of helical Yukawa rods

We consider the interaction potential between two chiral rod-like colloids which consist of a thin cylindrical backbone decorated with a helical charge distribution on the cylinder surface. For sufficiently slender coiled rods a simple scaling expression is derived which links the chiral `twisting' potential to the intrinsic properties of the particles such as the coil pitch, charge density and electrostatic screening parameter. To predict the behavior of the macroscopic cholesteric pitch we invoke a simple second-virial theory generalized for weakly twisted director fields. While the handedness of the cholesteric phase for weakly coiled rods is always commensurate with that of the internal coil, more strongly coiled rods display cholesteric order with opposite handedness. The correlation between the symmetry of the microscopic helix and the macroscopic cholesteric director field is quantified in detail. Mixing helices with sufficiently disparate lengths and coil pitches gives rise to a demixing of the uniform cholesteric phase into two fractions with a different macroscopic pitch. Our findings are consistent with experimental results and could be helpful in interpreting experimental observations in systems of cellulose and chitin microfibers, DNA and {\em fd} virus rods.

cond-mat.soft↗

arXiv2009

Generalized van der Waals theory for the twist elastic modulus and helical pitch of cholesterics

We present a generalized van der Waals theory for a lyotropic cholesteric system of chiral spherocylinders based on the classical Onsager theory for hard anisometric bodies. The rods consist of a hard spherocylindrical backbone surrounded with a square-well potential to account for attractive (or soft repulsive) interactions. Long-ranged chiral interactions are described by means of a simple pseudo-scalar potential which is appropriate for weak chiral forces of a predominant electrostatic origin. Based on the formalism proposed by Straley [Phys. Rev. A {\bf 14}, 1835 (1976)] we derive explicit algebraic expressions for the twist elastic modulus and the cholesteric pitch for rods as a function of density and temperature. The pitch varies non-monotonically with density, with a sharp decrease at low packing fractions and a marked increase at higher packing fractions. A similar trend is found for the temperature dependence. The unwinding of the helical pitch at high densities (or low temperatures) originates from a marked increase in the local nematic order and a steep increase of the twist elastic resistance associated with near-parallel local rod configurations. This contrasts with the commonly held view that the increase in pitch with decreasing temperature as often observed in cholesterics is due to layer formation resulting from pre-smectic fluctuations. The increase in pitch with increasing temperature is consistent with an entropic unwinding as the chiral interaction becomes less and less significant than the thermal energy. The variation of the pitch with density, temperature and contour length is in qualitative agreement with recent experimental results on colloidal {\em fd} rods.

cond-mat.soft↗

arXiv1999

Beam extraction studies at 900 GeV using a channeling crystal

Luminosity-driven channeling extraction has been observed for the first time in a 900 GeV study at the Fermilab Tevatron. This experiment, Fermilab E853, demonstrated that useful TeV level beams can be extracted from a superconducting accelerator during high luminosity collider operations without unduly affecting the background at the collider detectors. Multi-turn extraction was found to increase significantly the efficiency of the process. The beam extraction efficiency was about 25%. Studies of time dependent effects found that the turn-to-turn structure was governed mainly by accelerator beam dynamics. An investigation of a pre-scatterer using the accelerator flying wire system showed that a fiber could produce a significant extracted flux, consistent with expectations. Based on these results, it is feasible to construct a parasitic 5-10 MHz proton beam from the Tevatron collider.

physics.acc-ph↗

arXiv1994

Tevatron energy and luminosity upgrades beyond the Main Injector

The Fermilab Tevatron will be the world's highest energy hadron collider until the LHC is commissioned, it has the world's highest energy fixed target beams, and Fermilab will be the leading high energy physics laboratory in the US for the foreseeable future. Following the demise of the SSC, a number of possible upgrades to the Tevatron complex, beyond construction of the Main Injector, are being discussed. Using existing technology, it appears possible to increase the luminosity of the $\bar{p}p$ Collider to at least $10^{33}$cm$^{-2}$sec$^{-1}$ (Tevatron-Star) and to increase the beam energy to 2 TeV (DiTevatron). Fixed target beam of energy about 1.5 TeV could also be delivered. Leaving the existing Tevatron in the tunnel and constructing bypasses around the collider halls would allow simultaneous 800 GeV fixed target and $\sqrt{s}$ = 4 TeV collider operation. These upgrades would give Fermilab an exciting physics program which would be complementary to the LHC, and they would lay the groundwork for the construction of a possible post-LHC ultra-high energy hadron collider. (Presented at the Eighth Meeting of the Division of Particles and Fields Albuquerque, New Mexico, August 2-6, 1994.)

hep-ex↗